The present invention relates to an image forming apparatus for use as a color copying machine, a color printer and so forth and more particularly to an image forming apparatus for forming a plurality of monochromatic toner images and superimposing the plurality of monochromatic toner images on a transfer material in order to form a multicolor toner image.
Heretofore, an image forming apparatus of the sort mentioned above has, for example, four monochromatic toner image forming units for respectively forming a black, a yellow, a magenta and a cyan image, and a transfer material conveyer unit for conveying a transfer material such as printing paper. The four monochromatic toner image forming units are arranged in a line and the transfer material conveyer unit is adapted for conveying the transfer material along the arrangement of the monochromatic toner image forming units, so that the monochromatic toner image formed in each monochromatic toner image forming unit is successively transferred onto the transfer material.
Each of the monochromatic toner image forming units has, for example, a photoconductive drum which is rotatably installed, a uniform charger corotron for charging the photoconductive drum with a predetermined potential, a laser exposure unit for exposing to light the photoconductive drum according to image information, a developing device for developing a latent image with predetermined color toner, and a transfer corotron for transferring the toner image onto the transfer material. The monochromatic toner image forming unit concerned is used for forming the monochromatic toner image on the photoconductive drum by operating the uniform charger corotron, the laser exposure unit and the developing device while the photoconductive drum is being rotated. Further, the laser exposure unit has, for example, a rotatable polygon mirror and a light emitting element for irradiating the polygon mirror with exposure light from a predetermined direction. The light emitting element is caused to emit light according to the image information at timing at which the exposure light reflected from the polygon mirror is emitted onto the photoconductive drum, so that the latent image is formed on the photoconductive drum. The position between the photoconductive drum and the transfer corotron is hereinafter called a transfer position.
The transfer material conveyer unit has, for example, a transfer belt rotatably installed so as to pass the transfer position of each monochromatic toner image forming unit, a paper tray capable of accommodating a number of printing papers, a paper supply member for conveying printing paper from the paper tray to the transfer belt, a transfer-material drawing charger for electrostatically drawing the printing paper to the transfer belt, and a fixing device for fixing the transfer material peeled off the transfer belt.
In the image forming apparatus described above, the predetermined monochromatic toner image is formed in each monochromatic toner image forming unit and after the transfer material has been drawn to the transfer belt, the monochromatic toner image formed in each monochromatic toner image forming unit is successively transferred onto the transfer material in order to form a multicolor toner image.
In the case of such a conventional image forming apparatus, a monochromatic toner image of each color is formed in the corresponding monochromatic toner image forming unit and a multicolor toner image is formed by transferring the monochromatic toner image to the corresponding transfer position and superimposing the monochromatic toner images on the transfer material (transfer belt).
Consequently, in the image forming apparatus above, a color shift is caused between the monochromatic toner images of the multicolor toner image formed on the transfer material due to the shifting of the position where the toner image is exposed to light by the laser exposure unit on the photoconductive drum of each toner image forming unit and the shifting of the position where the toner image is placed with respect to the transfer position of each photoconductive drum. Thus, a change in hue, a color shift and the like are produced in the color image involved.
In the image forming apparatus above, moreover, not only the transfer position of the each photoconductive drum but also the exposure position in each toner image forming unit varies as the environmental conditions such as temperature and moisture change; the problem in this case is that the way the color shift occurs becomes unsettled.
In order to correct the color shift, it is considered to adjust the position where the monochromatic toner image is formed on the transfer material by controlling the timing, on a laser exposure unit basis, at which the image information is superposed on the exposure light so as to vary the position where the latent image is written to the photoconductive drum, that is, the timing at which the monochromatic toner image corresponding to the latent image reaches the transfer position.
However, it is still not possible to correct the position shift quantity of one exposure width or less, though it is possible to correct the color shift at each timing at which the reflective surface of each polygon mirror is rotated to a predetermined position, that is, to correct the color shift on a one exposure width basis through the technique mentioned above.
As disclosed in the Unexamined Japanese Patent Application Publication No. Hei 7-160084, further, the rotational phase of the polygon mirror used for the laser exposure unit of each monochromatic toner image forming unit is controlled, that is, the timing at which the reflective surface of the polygon mirror is rotated to a predetermined exposable-to-light position is controlled on a polygon mirror basis, in other words, the exposure timing is controlled on a polygon mirror basis so as to vary the timing at which the monochromatic toner image corresponding the exposure reaches each transfer position, whereby the position where the monochromatic toner image is formed on the transfer material is made adjustable. This technique is usable for correcting a color shift of one exposure width or less. On the other hand, the control of the rotational phase of the polygon mirror alone is disadvantageous for correcting the one exposure width or greater, so that this technique has to be used in combination of what has been mentioned previously.
In order to the control the rotational phase of the polygon mirror, however, a driving signal corresponding to each rotational phase needs preparing and besides a signal required to be synchronized with the rotation of the polygon mirror, for example, a timing signal (a so-called line sink signal) for superposing the image information on the light emitting element and the like has to be made to correspond to the rotational phase.
With the above-described control of the rotational phase of the polygon mirror, the greater the number of phase steps that can be set is made, the greater the number of various signal lines becomes, which makes it far delicate and difficult to design signal-line to signal-line synchronization and the adjustment thereof; 4 steps and even maximum 8 steps at the most.
Consequently, the rotational phase of the polygon mirror is controlled by dividing the one exposure width into 4 steps and even maximum 8 steps, with which the color shift is barely corrected within the resolution range, and a color shift of width less than that remains unadjustable and this has imposed limitation on the pursuit of a high-quality image.
Moreover, the control of the rotational phase of the polygon mirror makes it a premise to follow the technique of installing a polygon mirror in each monochromatic toner image forming unit and when each laser exposure unit emits exposure light to one polygon mirror, that is, when an image forming apparatus has only one polygon mirror to be commonly used for a plurality of monochromatic toner image forming units, the polygon mirror is unutilizable.
With the control of the rotational phase of the polygon mirror, further, when each laser exposure unit is designed to emit simultaneously more than one beam, for example, in the case of a so-called dural beam type laser exposure unit, a triple beam type laser exposure unit or a quad beam type laser exposure unit, the greater the number of simultaneous emission beams becomes, the greater the number of steps is required to be set, whereupon the aforementioned control is practically unable to be utilized.